1. Field of the Invention
The present invention relates generally to the field of aircraft manufacture, more specifically, to the manufacture of aircraft structures which have parts requiring the use of adhesives which must be cured in a vacuum under pressure at elevated temperatures.
2. Description of the Related Art
Aircraft wing sections or fuselage sections typically include an outer skin which is constructed of metal or composite material. The skin is reinforced from the back by a number of longitudinally extending stringers and laterally extending bulkheads or ribs. The stringers are received through openings in transversely extending bulkheads.
Fasteners such as rivets have been used to secure the stringers and bulkheads to the inside surface of the skin. Plastic-resin adhesives, e.g., carbon epoxies, have also been used.
These epoxies cannot properly bond at typical environmental temperatures or at atmospheric pressures but instead must be cured in an autoclave under negative pressure at elevated temperatures. The elevated temperatures are typically provided in an autoclave in which the skin is disposed on a tool. The upper surface of the tool may be flat, but oftentimes it is curved to accommodate contoured skin portions. To assemble the skin section, adhesive is applied between the skin and the stringer, and then the stringer is pressed against the skin with pressure in the desired orientation and location.
Before curing the epoxy adhesive, a non-porous release film and then a breather sheet are draped over the parts. The release film is a layer of film that is impermeable to and does not bond to the adhesive. Thus, it is able to contain the adhesive to keep it from sticking to the vacuum bag or other materials. The breather sheet is constructed of a loosely woven material, e.g., glass fabric, which serves as a continuous air path over the parts but does not come into contact with the epoxy.
The necessary vacuum pressure required is administered using a vacuum bag. A vacuum bag is a sheet of flexible transparent material which is initially draped over the parts which are to be adhered. Then the bagging material is sealed around its open edges using a tape or sealant. Thus, a vacuum can be created within the bag using a vacuum hose connected to a fitting on the bag.
The necessary pressure is then administered evenly to the outer surface of the bag. The uniformity of the pressure is dependent on a proper bag placement which will result in the bag engaging all of the surface area of the parts when the bag is pressurized. Because of this, proper bagging is critical to the curing process.
One thing that is problematic in this process is in the application of the bag to irregular or complex shapes, such as exist with a bulkhead/stringer intersection on the skin. Conventionally, this is accomplished by tucking the bag in to reach tight radii spots, corners, and other hard-to-reach areas. This is a time-consuming and arduous task. Employees have to meticulously place the bag with ample loose bag material around the stringers and bulkheads so that as a vacuum is drawn down on the bag, the bag is not stretched to rupture on a sharp edge. It is common for a large section having multiple stringers to take workers two hours to bag one section of structure.
Further, the task is so difficult that mistakes are common. Corners and other tight spots are oftentimes not reached by the bag because of improper bag placement, creating open spaces between the bag and the part surfaces. When this happens, air bubbles will become trapped in the epoxy during the cure creating voids. It may even cause the bag to rupture which prevents adherence of the parts.
There are also significant ergonomic and safety risks to employees having to crawl atop large tools to work the bag into corners and other complex areas.
Because of these disadvantages, there is a need in the art for a safe, efficient, and effective alternative.